1. Field of the Invention
The present invention relates to an image signal processing method for processing image signals detected by individual picture elements of a CCD (Charge Coupled Device) linear image sensor which is provided for reading an image of a document in an image reader.
2. Description of Related Art
In the image reader as mentioned above, a document is illuminated by a light source such as a halogen lamp or a fluorescent lamp and the reflected light therefrom is projected onto a CCD linear image sensor as a line image. Respective picture elements of the CCD linear image sensor receive the reflected light and convert it to individual image signals.
In a typical CCD linear image sensor, each picture element has such a small photosensitive area as is defined by 5.times.7 .mu.m.sup.2 and is aligned on a line at a pitch of 7 .mu.ms as shown in FIG. 17.
In order to read an image with a high resolution, it is desirable to make the size of each picture element smaller. However, such a smaller size for the picture element invites a shortage in the amount of light being incident thereinto upon reading a document, and therefore, it becomes impossible to obtain an effective image signal therefrom.
In principle, it is not impossible to obtain effective image signals in the CCD image sensor by concentrating the illuminating light emitted from the light source on a line having a width of several .mu.ms. However, in the case of a halogen lamp, the smallest width of the illuminating light is limited to an order of a few mms in the utmost because of limitations in positioning the filament in the halogen lamp and in fabricating the reflection mirror to be used for concentrating the illuminating light. In the case of a fluorescent lamp, the situation is worse than the case of the halogen lamp since the collection illumination with use of the former lamp is impossible in principle. Due to these reasons, it is very difficult to obtain a higher resolution of image even if an illuminating lamp having a high power is used.